Most commercial metal detectors operate in real-time including most handheld metal detectors designed to hunt for buried metal targets such as gold, coins, treasure and archaeological artifacts. Real-time means that output signal responds within a relatively short delay to any change in input signal; any delay being typically less than 1/10.sup.th of a second.
Most metal detectors have a transmitter whose output is connected to a transmit coil for the transmission of alternating magnetic fields, a magnetic field receiving means such as a receive coil which is connected to signal processing and assessment electronics. The signal processing and assessment electronics usually includes a preamplifier whose output is connected to synchronous demodulators whose synchronous demodulation multiplication functions are synchronised to the transmitted alternating magnetic fields. The outputs of the synchronous demodulators are connected to low-pass filters or “demodulation filters” whose outputs are further processed for target identification and indication.
Some commercially available sinusoidal single-frequency transmitting metal detectors have switches which allow a user to select different frequencies. The electronics in such detectors is often relatively expensive. The ability to select different frequencies may be useful, for example, in gold nugget prospecting where the size of gold nuggets may vary from location to location and hence the optimal frequency for detection may also correspondingly vary.
U.S. Pat. No. 5,537,041 discloses a metal detector which transmits multi-period pulses and operates in the time-domain, as too do some commercially available pulse induction metal detectors; see for example U.S. Pat. No. 4,868,504 and U.S. Pat. No. 5,576,624. Time-domain detectors are claimed to be relatively susceptible to electromagnetic interference owing to wide receiver bandwidths and extensive Fourier components of the synchronous demodulation multiplication functions, which in most cases have just 3 values; 0, +1 or −1.
U.S. Pat. No. 4,628,265 discloses a frequency-domain metal detector which applies a voltage square-wave signal to a transmit coil and the received fundamental and third harmonics are band-pass filtered and then synchronously demodulated. With this approach, only two frequencies are detected simultaneously owing to the narrow receive bandwidths of the band-pass filters.
U.S. Pat. No. 4,942,360 discloses a metal detector which detects more than one frequency simultaneously and operates in the frequency-domain. This is useful to reject magnetic soil signals and also to characterize targets. U.S. Pat. No. 5,506,506 discloses further steps to characterize targets in magnetic soils.
Some commercially available metal detectors use digital signal processing technology for sine-wave (and cosine) synchronously demodulation multiplication, as is also disclosed US2005/0253711. At least one of these commercial detectors also applies a square-wave voltage to a transmit coil and also detects the fundamental and third harmonic in the frequency-domain as does U.S. Pat. No. 4,628,285 but achieves this by sine-wave synchronously demodulation multiplication rather than exploiting the use of band-pass filters. The two systems in terms of signal-to-noise ratio are mathematically identical.
Some systems for detecting various geological profiles (such as water, salt, minerals) transmit a digital pulse width modulated voltage switching sequence with frequency components rich at various frequencies. These systems do not operate in real-time; that is a sequence record of data is first collected and then later processed after the collection. A complex fast Fourier transform is applied to the receive waveform recorded data from which a complex frequency profile of the environment may be indicated at the various rich transmit frequencies.
U.S. Pat. No. 6,686,742 patent discloses a metal detector wherein the synchronous demodulation multiplication functions are weighted more when the receive signal is relatively high, and weighted less when the receive signal is relatively less. This results in improved signal-to-noise ratio, particularly for fast time constant targets.
The advantage cited or assumed advantage for utilizing narrowband synchronous demodulation, such as sine-waves or band-pass filtering followed by synchronous demodulation multiplication, or by using complex fast Fourier transforms, all of which will yield the same signal-to-noise ratio, is that these techniques reduce both broad-band electronic noise and environmental magnetic interference.
An object of the current invention is to alleviate at least some of the problems of the prior art by offering an improved metal detector platform capability, or at least to provide the public with a useful alternative to known detectors.
A further object of this invention is to provide a real-time flexible metal detector platform improvement at relatively low cost that assists an operator such as a prospector in the location of metal targets.